DC to DC converters utilize a power switching transistor that is connected to a transformer or inductor to derive the appropriate output voltage(s). When the transistor tums off, drain-to-source current reduces from a high value to zero, the drain-to-source or collector-to-emitter voltage rises sharply due to the energy stored in the inductance. This high voltage can result in drain-to-source or collector-to-emitter junction breakdown in the transistor, which can destroy the transistor.
A snubber is used to prevent junction breakdown of the switching transistor. With reference to FIG. 1, a conventional snubber 130 is made up of resistor 120 connected in series with capacitor 115. It is connected across the drain-source terminals of FET 111. When FET 111 tums off, the voltage rise between the drain (D) and source (S), caused by inductive load 113, is controlled by the RC network of resistor 120 and capacitor 115. The voltage is prevented from reaching damagingly high levels because the voltage rise on capacitor 115 is dictated by the RC load.
In conventional designs, pulse width modulator (PWM) 121 and gate driver 112 have their own power supply 117 (shown by zener 117). Current is provided by resistor 114 into the power supply 117 to power the PWM and gate driver. This means energy is dissipated as heat in resistor 114.
An improvement of the conventional method is disclosed in U.S. Pat. No. 4,414,479 issued to James Foley on Nov. 8, 1983. Foley's figure is duplicated in FIG. 2 with the elements renumbered. He powers his base driver 212 with charge stored in snubber capacitor 215. This is an improvement since the switching transistor 211 does not have to discharge capacitor 215 when it tums on, instead this energy is used to power the base driver 212. The base driver 212 needs no external power supply. This improves efficiency of the switching transistor. This circuit, however, has a few shortcomings. In an AC to DC power supply (commonly called an off-line switcher), AC voltage is rectified to a DC voltage (for 120 Vac this is approximately 170 Vdc). The base driver 212 and the PWM (not shown) power supplies are typically between 5-15 Vdc. When transistor 211 is off, resistor 216 has about 160 Vdc on it, assuming power supply 217 is a 10 V power supply. This means for low power loss in resistor 216, its value has to be high. If resistor 216's value is high, not enough current can be pumped into the power supply such that a PWM and the base driver 212 can both be powered using snubber power. Thus, Foley can only provide a limited increase in power efficiency.